Portable communication devices, such as cellular telephones, portable computers, personal digital assistants (PDAs), and the like, are configured to communicate over wireless networks. Such portable communication devices may enable communication over multiple networks, each of which has corresponding transmit and receive frequency bands within a composite broadband frequency range. Depending on design requirements, the frequency bands may have large spectrums and/or may be separated from one another by a significant range of frequencies. For example, the composite broadband frequency range may span from about 1700 MHz to about 2170 MHz, and may include multiple frequency division duplex (FDD) frequency bands of networks over which a communication device is able to transmit and receive radio frequency (RF) signals, such as band 1 (uplink 1920-1980 megahertz (MHz); downlink 2110-2170 MHz), band 2 (uplink 1850-1910 MHz; downlink 1930-1990 MHz), band 3 (uplink 1710-1785 MHz; downlink 1805-1880 MHz), band 4 (uplink 1710-1755 MHz; downlink 2110-2155 MHz), and band 25 (uplink 1850-1915 MHz; downlink 1930-1995 MHz). High band filters may additionally support FDD LTE bands (e.g., B30 and B7) and time division duplex (TDD) bands (e.g., B40, B41).
To provide filtering of the RF signals in a composite broadband frequency range requires an ultra-wide passband for transmitting and receiving the full range of frequencies. Ultra-wide bandwidth band pass filters are therefore needed to accommodate the large passbands. An ultra-wide bandwidth may be considered any bandwidth in excess of eight percent of a center frequency fcenter.
Various types of band pass filters may be used in communication devices, including LC filters reliant on inductors and capacitors, and acoustic filters reliant on acoustic resonators. The acoustic resonators may include surface acoustic wave (SAW) resonators or bulk acoustic wave (BAW) resonators, for example, where the BAW resonators may include thin film bulk acoustic resonators (FBARs) and/or solidly mounted resonators (SMRs). Generally, LC filters are able to provide very wide bandwidths. However, LC filters do not provide sufficiently steep roll-off for corresponding passbands at the very wide bandwidths, with acceptably low insertion loss for efficient operation. Roll-off is a filter characteristic indicating how quickly the transition occurs between the filter passband and the filter stop band on either side of the passband, where the steeper the roll-off, the more efficient the transition. Generally, the steepness of the roll-off increases (i.e., thus improves) using higher order filters. However, the higher the order of an LC filter, in particular, the greater the insertion loss. However, acoustic filters are not able to accommodate sufficiently wide bandwidths, e.g., due to limited intrinsic acoustic coupling, to be suitable for use as very wide bandwidth filters.
Long Term Evolution (LTE)-Advanced is mobile communication standard that includes a carrier aggregation (CA) feature. CA involves combining, or aggregating, multiple component carriers of multiple respective frequency bands to attain a greater total transmission bandwidth. Multiple filters having different pass bands may be combined in a multiplexer (MUX) of a portable communications device to perform CA. Such MUXes provide frequency division multiplexing of multiple signals having respective frequencies that fall into the respective pass bands of the respective filters. A MUX allows the signals to be simultaneously transmitted (uplink) from the portable communications device over the respective frequencies of the respective pass bands. The MUX also performs demultiplexing to separate signals having the respective frequencies that are received (downlink) by the portable communications device.
FIG. 1 illustrates a schematic diagram of a typical MUX 2 made up of three LC filters 3, 4 and 5, each of which is made up of a particular configuration of inductors 6 and capacitors 7 that achieves a respective pass band. All of the LC filters 3, 4 and 5 are connected to an antenna 8 of the portable communications device (not shown). In this example, LC filter 3 provides a low pass band ranging from 700 to 960 MHz, LC filter 4 provides a middle pass band ranging from 1710 to 2170 MHz, and LC filter 5 provides a high pass band ranging from 2300 to 2690 MHz. The frequency gap between the upper edge of the middle pass band (2170 MHz) and the lower edge of the high pass band (2300 MHz) is only 130 MHz. Due to this small frequency gap, adjacent frequency bands in the middle and high pass bands should be sufficiently attenuated to prevent the adjacent bands from overlapping. This requirement, however, is difficult to achieve with acceptable insertion loss using LC filters of the type shown in FIG. 1, as will now be explained with reference to FIGS. 2A and 2B.
FIG. 2A is a graph containing first, second and third frequency response plots 11, 12 and 13 for the LC filters 3, 4 and 5, respectively, shown in FIG. 1. FIG. 2B is an enlarged view of the portion of the graph contained within the dashed block 14 shown in FIG. 2A. With reference to FIG. 2B, it can be seen that signal loss at the upper edge at 2170 MHz (reference numeral 15) of the middle pass band (plot 12) is around 4 decibels (dB), and that signal loss at the lower edge of the high pass band (plot 13) at 2300 MHz (reference numeral 16) is around 4 dB. This amount of attenuation at these adjacent edges of the middle and high pass bands is insufficient to ensure that overlap between the pass bands is avoided. Furthermore, if there is an additional rejection requirement, such as where an LC filter is included that provides a GPS/GNSS/Beidou pass band (1560 to 1606 MHz), then there is only a 104 MHz frequency gap between the lower edge of the middle pass band and the upper edge of the GPS/GNSS/Beidou pass band (not shown). It can be seen in FIG. 2B that the lower edge of the middle pass band at 1710 MHz (reference numeral 17) is around 4 dB, which is an insufficient amount of attenuation at the adjacent edges of these bands to prevent overlap.
In addition, the MUX 2 shown in FIG. 1 exhibits a relatively high insertion loss in the middle and high pass bands. Because the MUX 2 is typically placed directly at the antenna 8, it generally is unsuitable for use in today's portable communications devices because its high insertion loss would lead to very poor system efficiency.
A need exists for an ultra-wide bandwidth MUX for use in a portable communications device that has low insertion loss and that provides sufficient attenuation at adjacent edges of adjacent pass bands.